VS: Add support for C++ module internal partitions in VS 17.6 and newer

VS 17.6 now implements `ScanSourceforModuleDependencies` using the same
`cl /scanDependencies` scanner that our Ninja generator uses.  It can
distinguish module internal partitions from module interface units based
on their content.  Switch from `CompileAsCppModule` to `CompileAsCpp`
for `CXX_MODULES` sources so that MSBuild can scan and classify them.

.gitlab/ci/configure_windows_vs2022_x64.cmake

@@ -1,4 +1,4 @@
-set(CMake_TEST_MODULE_COMPILATION "named,partitions" CACHE STRING "")
+set(CMake_TEST_MODULE_COMPILATION "named,partitions,internal_partitions" CACHE STRING "")
 
 include("${CMAKE_CURRENT_LIST_DIR}/configure_windows_msvc_cxx_modules_common.cmake")
 include("${CMAKE_CURRENT_LIST_DIR}/configure_windows_vs_common.cmake")

Source/cmGlobalVisualStudio10Generator.h

@@ -149,6 +149,8 @@ public:
 
   virtual bool IsUtf8EncodingSupported() const { return false; }
 
+  virtual bool IsScanDependenciesSupported() const { return false; }
+
   static std::string GetInstalledNsightTegraVersion();
 
   /** Return the first two components of CMAKE_SYSTEM_VERSION.  */

Source/cmGlobalVisualStudioVersionedGenerator.cxx

@@ -739,6 +739,22 @@ bool cmGlobalVisualStudioVersionedGenerator::IsUtf8EncodingSupported() const
           cmSystemTools::VersionCompareGreaterEq(*vsVer, vsVer16_10_P2));
 }
 
+bool cmGlobalVisualStudioVersionedGenerator::IsScanDependenciesSupported()
+  const
+{
+  // Supported from Visual Studio 17.6 Preview 7.
+  if (this->Version > cmGlobalVisualStudioGenerator::VSVersion::VS17) {
+    return true;
+  }
+  if (this->Version < cmGlobalVisualStudioGenerator::VSVersion::VS17) {
+    return false;
+  }
+  static std::string const vsVer17_6_P7 = "17.6.33706.43";
+  cm::optional<std::string> vsVer = this->GetVSInstanceVersion();
+  return (vsVer &&
+          cmSystemTools::VersionCompareGreaterEq(*vsVer, vsVer17_6_P7));
+}
+
 const char*
 cmGlobalVisualStudioVersionedGenerator::GetAndroidApplicationTypeRevision()
   const

Source/cmGlobalVisualStudioVersionedGenerator.h

@@ -44,6 +44,8 @@ public:
 
   bool IsUtf8EncodingSupported() const override;
 
+  bool IsScanDependenciesSupported() const override;
+
   const char* GetAndroidApplicationTypeRevision() const override;
 
   bool CheckCxxModuleSupport() override

Source/cmVisualStudio10TargetGenerator.cxx

@@ -2802,7 +2802,14 @@ void cmVisualStudio10TargetGenerator::OutputSourceSpecificFlags(
          fs->GetType() == "CXX_MODULE_HEADER_UNITS"_s)) {
       if (lang == "CXX"_s) {
         if (fs->GetType() == "CXX_MODULES"_s) {
-          compileAsPerConfig = "CompileAsCppModule";
+          if (shouldScanForModules &&
+              this->GlobalGenerator->IsScanDependenciesSupported()) {
+            // ScanSourceforModuleDependencies uses 'cl /scanDependencies' and
+            // can distinguish module interface units and internal partitions.
+            compileAsPerConfig = "CompileAsCpp";
+          } else {
+            compileAsPerConfig = "CompileAsCppModule";
+          }
         } else {
           compileAsPerConfig = "CompileAsHeaderUnit";
         }